Many modern illumination sources are dimmed by means of a digital dimming signal as the input signal controlling the dim level, rather than by a traditional analogue signal from a potentiometer or the like. The digital dimming signal may for example originate from an automated lighting controller, or from a manual user input entered through a suitable user input device such as a digital wall switch or wall panel or a lighting control app running on a user terminal such as a tablet, smartphone or smart watch. For instance, a lighting controller may be configured to automatically control the illumination level to increase gradually from zero or a very low level up to the maximum or at least a higher level, starting from and/or ending at a certain user defined time. If the light source is located in the bedroom, this can be used to provide the user with a wake-up lighting effect that wakes the user up gradually, in a more natural way than the abrupt wake-up experience of a traditional audio alarm.
In general an illumination source may comprise a single light-emitting element whose output intensity can be varied, or an array of individually-addressable light-emitting elements whose output intensities can each be varied individually. The light-emitting element (or each light-emitting element) may take any of a variety of possible forms, such as an LED or cluster of LEDs, a filament bulb, or a fluorescent tube. For example the illumination source may take the form of an LED-based lamp comprising either (I) a set of LEDs that can only be controlled together as a whole (as a single light-emitting element), or (II) a “pixelated” array of LEDs or subgroups of LEDs which can be controlled individually. In the case of a single or a given light-emitting element, the output intensity level can be varied by varying the current through the light-emitting element, and/or by means of pulse width modulation (PWM) whereby the length of a periodic pulse is varied so as to vary the average current over multiple periods.
If the digital dimming signal is n bits in length, this means there are 2n possible output intensity levels that can be represented in the input. As the input dimming signal varies over the 2n different values, a digital controller controls a driver to convert the 2n digital values into 2n different output levels in the intensity of the illumination emitted by the illumination source, e.g. by varying the current or PWM duty cycle. However, the ability of the illumination source to render 2n different illumination values might be limited by the nature of the light-emitting element(s), the driver circuitry, and/or one or more other components of the illumination source, or a combined effect of two or more of these. If the degree of imprecision due to such factors is greater than the digital step-size in the input dimming signal, then the illumination source will be unable to usefully realize as many levels as the input signal can represent. Put another way, the resolution of the illumination source itself may be equivalent to less than n bits. Hence either the illumination source cannot render the step size of the digital dimming signal, or else the number of bits n in the digital dimming signal is simply designed to be no greater than the equivalent bit-resolution of the illumination source.
One effect of the above is that the lowest achievable dimming level may be relatively high. This can be especially problematic given that humans experience light intensity on a logarithmic scale, and are therefore more sensitive to a given step size at a low light level compared to their sensitivity to the same step size at a higher light level. Particularly, the user will be very sensitive to the first step between zero (no illumination) and the first non-zero dimming level. This can be problematic for instance if the illumination is being used to provide gradual wake-up lighting, because, if the resolution is too coarse, then the first step between zero and the first non-zero illumination level may cause the user to wake suddenly instead of gradually, thus obviating the intended effect.
To address the issue of reducing the lowest non-zero dimming level, U.S. Pat. No. 8,803,446 discloses a spatial dimming technique whereby the individually-controllable light-emitting elements of an array are controlled to emit in a spatial pattern that varies as a function of a dimming ratio indicated in the input dimming signal. According to U.S. '446, a dimming control unit is configured so as, when the dimming ratio falls within a first dimming range, to vary the power supply to the light source in accordance with the dimming ratio; but when the dimming ratio falls in a second dimming range, to vary the number of light-emitting elements that are lit.
By making use of spatial dimming, it is possible to achieve “deep dimming” whereby finer steps in the overall emitted illumination level are achievable, including a smaller step size between zero (no illumination output) and the lowest no-zero illumination output level.